The present invention relates to a heat roller fixing device used in a printing apparatus which performs development for printing after forming an electrophotographic or electrostatic latent image and, more particularly, to a heat roller fixing device which allows easy mounting and demounting of a heat roller unit.
Printing apparatuses (e.g., a copy machine, a recording apparatus, and the like) utilizing an electrophotographic technique or an electrostatic recording technique have been widely used. In such an electrophotographic printing apparatus, a latent image is formed and is converted to a visible image using a developer (which normally adopts a developing agent powder). Thereafter, the powder image (toner image) is fixed using a fixing device.
In this electrophotographic apparatus, a heat roller fixing device having a simple structure is adopted, and the heat roller fixing device of this type has a long life, is fixed to the apparatus, and cannot be easily removed therefrom.
Namely, in the conventional apparatus, the service life of the heat roller corresponds to 100,000 sheets or more, and the halogen lamp (heat source) incorporated therein also has a long life. When the service life of the heat roller fixing device ends, it is considered that the life of the apparatus is also ended. Therefore, the fixing device is arranged so that it cannot be replaced.
However, as the service life of such an apparatus has been prolonged by advanced techniques, a demand has arisen for an arrangement allowing the replacement of the heat roller type fixing device and, more specifically, the roller and halogen lamp of the heat roller. In the related art device, replacement and maintenance are difficult.
In the heat roller fixing device in which the fixing is carried out by the heat roller, the temperature of the heat roller is measured, and accordingly, the power source of the heater which is used as a heat source is controlled. In the detection of the temperature to lessen the temperature ripple, a thermistor having a small capacitance and a negative temperature characteristic is provided, namely, the resistance becomes small when the temperature rises.
Therefore, if an apparent resistance of the thermistor becomes large because of a failure of the thermistor, it is judged that the temperature of the heat roller is low, and the power supply to the heater source is continued, which causes a runaway overheating of the heater.
On the other hand, in the construction of the heat roller fixing device, the temperature detecting portion is connected via a connector to a main body, and the above-mentioned heat runaway overheating can be caused when the connector is disconnected. Accordingly, and desirably, supply of the power source for the heater by detecting a disconnection of the connector is prevented.
To detect a disconnection of a connector or a break down of a thermistor, conventionally a method is adopted whereby a variation of a resistance of the thermistor is detected during a predetermined constant period from the time when the power supply for the heater is commenced.
As explained above, the temperature detecting portion is connected to the main body, and this connection is often forgotten when the apparatus is mounted or when maintenance or testing is carried out. Confirming the connection is carried out by an indirect method, such as checking a variation of the resistance of the thermistor, and therefore, it is difficult to prevent runaway overheating, and thus operation of the heat fixing apparatus, becomes unreliable.